Hydride Is Not a Spectator Ligand in the Formation of Hydrido Vinylidene from Terminal Alkyne and Ruthenium and Osmium Hydrides: Mechanistic Differences

RuHX(H 2 )L 2 (L = P t Bu 2 Me; X = Cl, I) and OsH 3 ClL 2 (L = P i Pr 3 ) react (time of mixing) with terminal alkynes in a 1:2 stoichiometry to give MHX( C CHR)L 2 and RCH CH 2 . The reactions of RuHX(H 2 )L 2 with PhC⋮CD lead to RuDX( C CHPh)L 2 as the only isotopomers. This result is understood in terms of an insertion of the alkyne in the Ru−H bond to make a vinyl, followed by an α-hydrogen migration, giving the hydrido/vinylidene. This reaction path has been studied by ab initio (B3LYP) calculations with location of the minima and transition states. In agreement with isotopic labeling experiments, it is shown that the path starting with insertion into the Ru−H bond is more favorable than a 1,2-migration within the coordinated alkyne ligand. The favored reaction is calculated to be exothermic from the starting compounds to intermediates and to products with the energies of all transition states well below that of the separated reactants. The mechanism involves the formation of a 14-electron η 1 -vinyl intermediate which, after structural changes, gives the 16-electron hydrido/vinylidene product. The same general reaction path is calculated to be preferred for Os, but differences occur between the geometries of the vinyl intermediate (it is η 2 -vinyl for Os) and in the total number of intermediates involved.